.30cal (.30 caliber)is a variation of ammunition in Return to Castle Wolfenstein. It is exclusively used by the M1S Snooper Rifle. You can carry up to 15 rounds (5 in magazine and 10 in reserve). There are 3 types of .30cal: a single magazine, two magazines and an ammo box. The one magazine gives you 2 rounds, the double gives you 5 rounds, and the ammo box gives you 10 rounds.
This is the rarest type of ammo in the game. So use it wisely.